If you are thinking of storing items in your garage or if you have electrics in there then it is important that it is watertight. It may have been watertight when it was built but if over time the roof has developed a leak or the bricks have holes in to the outside then you may find that you get water starting to come in.
This can cause all sorts of problems and can result in irreversible damage. If you have items stored in your garage such as clothing or rugs etc, then getting them wet could cause them to smell damp and eventually grow mould. Mould can grow on a number of different items and so you need to keep your eye out for it as it is very common in a garage.
If you want to turn your garage in to a gym or a playroom then you will definitely need to ensure that it is water tight and you may also want to install central heating and insulation. It may be that you need to ask for planning permission to make some of the changes needed in order to compete the project. All of this adds extra cost and time so it is important to fully understand what is involved before you start.